Strange VB6 build problems (related to nlog)

Question

This I think is related to my use of the nlog C++ API (and my question on the nlog forum is here); the purpose of my asking this question here is to get a wider audience to my problem and perhaps to also get some more general ideas behind the VB6 IDE's failure to build in my particular scenario.

Briefly, the problem that I am having is that I am having trouble building VB6 components which reference unmanaged C++ components which have calls to nlog's C\C++ API (which is defined in NLogC.DLL). The build problems are not occurring during compile time, they are occurring when the binary is being built which suggests to me that it's some kind of linker type problem? Don't know enough about how VB6 binaries are produced to tell. The VB6 binary is produced, but it is corrupted and crashes shortly after it is invoked.

Has anyone had any similar experiences with VB6 (doesn't have to be related to nlog or C++)?

edit: Thanks for all the responses to this rather obscure problem. Still no headway unfortunately; my findings since I posted this:

1. 'Tweaking' the compile options doesn't appear to help in this problem.
2. Adding a reference to the nlog-enabled C++ component from a 'blank' VB6 project doesn't crash it or cause weird build problems. So it isn't a 'native' VB6 issue, possibly an issue with the interaction between nlog and the various components and 3rd party libraries used by other referenced components?
3. As for C++ calling conventions: the nlog-enabled C++ component is - as far as I can see - compliant to these conventions and indeed works fine when referenced by VB6 as long as it is not making any nlog API calls. Not sure if the nlogc.DLL itself is VB6 compliant but I would have thought that that is immaterial since the API calls are being made from the C++ component; VB6 shouldn't know or care about what the C++ component is referencing (that's as far as my understanding on this goes...)

edit2: I should also note that the error message obtained during build is: "Errors during load. Please refer to "xxx" for details". When I bring up the log file, all that there is in there is: "Cannot load control xxx". Interestingly, all references to that particular control disappears from that particular project resulting in compile errors if I were to try to build again.

Answer 1

I would try tweaking some of the Compile options found in the Project, Properties menu, Compile panel to see if they yield any additional hints as to what is going wrong.

For example if you compile the executable to p-code rather than native code does it still crash on startup.

Answer 2

What error message do you get when you run your compiled binary?

I doubt the compiler/linker is the problem: project references in a VB6 project are not linked into the final executable. A project reference in VB6 is actually a reference to a COM type library (which may or may not be embedded in a .dll or other binary file type). Project references primarily serve two purposes:

1. The IDE extracts type information from the referenced type libraries which it then displays in the Object Browser (and in the Intellisense drop-down)

2. At compile-time, the compiler extracts the type information stored in the referenced libraries, including the CLSID of each class that you instantiate, and embeds this data into the executable. This allows your executable to create instances of classes contained in the libraries that you referenced.

Note that the compiled binary doesn't link to any code in the referenced libraries, and it doesn't even contain the filenames of the referenced libraries. The final executable only contains the CLSID's and other type information that it needs to instantiate COM objects at run-time.

It is much more likely that the issue is with NLog, or with how you are calling it from your code, rather than something gone awry in the VB6 compile process.

Answer 3

If you think it might be a linker problem, this should crash it the same way:

1. create a new standard project (of any kind)
2. add a new module and copy the "declare"-statements into it
3. compile

If it doesn't crash it is something else.

Answer 4

It would help an exact description of the error or a screenshot of what going on.

One thing to check is wherever NLogC.DLL or the C++ DLL you built have the correct calling convention defined. Basically you can't have the DLL function names mangled or use anything but the STDCALL calling convention. If the C++ DLL has not been created with those two things in mind then it will fail to work with VB6.

MSDN Article on Calling convention.

Answer 5

Got around the problem by using NLog's COM interface (NLog.ComInterop.DLL) from my unmanaged C++ code. Not as easy to do as the C\C++ API but at least it doesn't crash my VB6 components.

Answer 6

"Cannot load control xxx" errors can be caused by .oca files which were created from a different version of an .ocx than currently used. If that is the case, deleting the .oca files helps.